The present invention relates to a general process for the manufacture of sulfenamides.
2-benzothiazole sulfenamides have become commercially important as rubber vulcanization accelerators, lubricating oil additives, and fungicides. Their most important commercial utilization is their use as accelerators in rubber vulcanization; in order to have efficient application in such a process, the sulfenamides accelerators must be of high purity.
The preparation of sulfenamides is well known in the prior art. The most widely used process involves the oxidation of 2-mercaptobenzothiazole (called MBT) in the presence of the desired amine. The oxidation agent can be sodium hypochlorite, a halogen, hydrogen peroxide, potassium persulfate, potassium dichromate, etc. The prior art processes for making the sulfenamides are normally complicated or produce a crude product that must be further purified. U.S. Pat. No. 3,600,398 teaches the use of a molar ratio of MBT to the amine in the range of at least 1:8 to 1:10 ratio in order to obtain the sulfenamide in pure form in excellent yield. In the present invention, surprisingly, a ratio of 1:4 gave a 91.4% pure product while a ratio of 1:5 afforded a 99.4% pure sulfenamide. More surprisingly, when the ratio of MBT to amine was raised to 1:7, the purity of the product fell to 98% (see Table II d, e, f).
Although part of the amine could be recovered according to known procedures, the less amine used the more economical the process will be. U.S. Pat. No. 2,419,283 teaches the oxidation process for making the sulfenamides by using additional material such as Na.sub.2 SO.sub.4 or NaCl which also produces impurities in the final product. British Pat. No. 772,582 teaches a complex oxidation method for producing the sulfenamides that reuires constant pH correction, addition of NaOH and the determination of potential. British Pat. No. 655,668 teaches an oxidation method limited to primary amines and an excess of water. Example 14 illustrates the failure of this process when the secondary amine, morpholine, and a comparatively large amount of water were used. No product could be isolated from this process.
The present invention overcomes the above mentioned problems of the prior art processes since there is no addition of strong alkaline solutions or strong acids that will create a need for the adjustment of the pH. The present invention involves a general process which is not limited to primary amines yet it is a simple and inexpensive process that does not require many steps. By using this simple process, it is possible to prepare most of the sulfenamide derivatives, if not all, which previously were prepared by distinct individual processes. Furthermore, the fact that these sulfenamides are produced in high yields and purity without the need for purification is an important advantage of this invention.